1. Field of the Invention
The present invention relates to a signal processing method and a solid-state image sensing device for converting pixel voltage signals into digital signals.
2. Description of the Related Art
Analog signals read from a pixel matrix are converted into digital signals by an A/D converter. An analog signal read from a pixel has a voltage level that is represented by the sum of a pixel-resetting component (black level) and the pixel's own signal component (i.e., a signal component of the pixel itself).
Japanese Laid-Open Patent Publication No. 2009-296423 discloses an integrating A/D converter for producing a signal component from a pixel by finding the difference between a black level and a signal level. More specifically, the disclosed integrating A/D converter counts down a train of successive pulses depending on the black level, and thereafter counts up a train of successive pulses depending on the signal level, thereby producing a digital pixel signal level. When counting pulses, the disclosed integrating A/D converter counts down the train of successive pulses depending on the black level W times, and counts up the train of successive pulses depending on the signal level W times, thereby reducing noise caused by the A/D conversion process.
According to the technology disclosed in Japanese Laid-Open Patent Publication No. 2009-296423, however, since the bits of the counter are expanded, the train of pulses depending on the resetting component and the train of pulses depending on the signal level need to be repeatedly read and counted the same number of times, and thus the time required by the A/D conversion process is increased. Furthermore, depending on the number of times that pulses are repeatedly counted, the gradient of a ramp waveform is made sharper in order to minimize any increase in time required by the A/D conversion process. However, such an approach tends to result in increased quantization noise.